CD137 is an inducible molecule of the tumor necrosis factor (TNF) receptor superfamily expressed on activated T cells, NK cells and dendritic cells. Our and others' laboratories have demonstrated that CD137 agonistic monoclonal antibodies (mAb) costimulate potent cell-mediated immune responses leading to regression of established tumors in animal models. Our Iong-term goal is to elucidate the mechanisms of the CD137 mAb in immune potentiation, to maximize its therapeutic potential, and to brinq CD137 mAb-based new therapeutics to treat human cancers. We have recently found that CD137 mAb could reverse established tolerance/anergy of T cells in vivo. This may explain, at least in part, its role in overcoming tumor-induced T cell tolerance. Many human cancers express high levels of B7-H1 and/or B7-H4, which are capable of down-regulating T cell immunity. Therefore, the therapeutic effect of CD137 mAb may be compromised. In addition, we believe there are sufficient data at this time to support the development of anti-human CD137 agonistic mAb for future clinical trials of cancer therapy. In this proposal, we will elucidate the mechanisms of CD137 mAb in the reversal of tumor-induced T cell anergy and evaluate the method and approaches to overcome the roles of tumor-associated B7-H1 and B7-H4 in the evasion of tumor immunity as approaches to maximize therapeutic efficacy. Finally, we will establish "humanized" CD137 mouse model using gene knock-in technology to facilitate the selection of anti-human CD137 mAb for future clinical trials. Our studies will have a direct impact on the future application of CD137 mAb in cancer treatment.